HEAT networks Storage Modules
HEAT networks can use a standardized storage module that is 30 metres in diameter and 200 metres deep. The module basically consists of 48 plastic pipes buried in the ground. At the surface there is an air-to-ground heat exchanger that is similar to a large car radiator. The fan for the radiator runs quietly and at a low speed because the rate of heat exchange is quite modest, and in fact the fan might in future be dispensed with altogether, relying instead on natural breezes to move the air through the radiator. The size of the module is determined by the rate of heat flow through the ground (or rock). Heat collected in the summer and injected near the center of the storage field takes six months to travel to the periphery, where it is extracted. This provides for the option of using a single storage module for both heat and cold where that is desirable. The modules can be packed closely together, and they can be located under parks, parking lots, streets, etc., without interfering with the normal uses of the land. The lifetime of the plastic pipes is at least 50 years, based on experience with ground source heat pump heat exchangers, which are basically the same.
The maximum capacity of the storage module is 1.5 million kWh, consisting of 750,000 kWh for each of the winter and summer seasons in cases in which the seasonal energy extraction modes are balanced. Because of the heat transport delay the storage module can simultaneously accumulate cold to be used in the summer while it is delivering heat in the winter. In some cases a module may not need to operate for both seasons, in which case its capacity is limited to 750,000 kWh, but it will be easier to recharge because some storage will normally carry over from one year to the next.
The power delivery is approximately 500 kW per module, or enough to heat 50 homes. New housing starts in Ontario in 2004 were 85,114 units. Over a 20 year period the number of new homes that might use HEAT networks could amount to 1.7 million homes, based on a constant construction rate. That would require 34,000 modules, producing a peak thermal power of 17,000 megawatts (thermal) for new homes. Retrofitting to older homes and more especially fitting HEAT networks to office buildings and shops (which can be easily accomplished) should roughly double that total, bringing it to roughly 34,000 megawatts for heating. The overall cooling demand is lower. From the annual power demand curves it can be observed to amount to about 9,000 megawatts (electrical) at the present time, growing to an estimated 12,000 megawatts.
These two power demands are in different physical forms, and they occur at different times of the year, but for the purposes of comparing them to other sources of energy they can be added together. At 46,000 megawatts HEAT networks are capable of delivering much more energy than existing or planned nuclear reactors, or hydro dams, etc. Moreover, their energy delivery is extremely reliable and stable, they do not produce carbon dioxide or other hazardous byproducts, and they inherently offer a large excess capacity to handle unusual demands.
Bearing in mind that at 30m the storage fields are quite small, the construction of an average of 1700 storage modules per year across Ontario should not present much of a challenge.